Simplified bottom electrode-barrier structure for making a ferroelectric capacitor stacked on a contact plug

ABSTRACT

The present invention is related to the realization of a simplified bottom electrode stack for ferroelectric memory cells. More particularly, the invention is related to ferroelectric memory cells wherein the ferroelectric capacitor is positioned directly on top of a contact plug. The bottom electrode stack is prepared by depositing a ferroelectric film atop an Ir or Ru metal electrode layer, then annealing the ferroelectric layer in an oxygen ambient wherein the partial pressure of oxygen is controlled at a level sufficient to oxidize the ferroelectric layer but not at a level sufficient to oxidize the metal electrode layer

RELATED APPLICATION

This application is a divisional of U.S. Ser. No. 10/292,363, filed Nov.8, 2002, which claims priority under 35 U.S.C. § 119(e) to U.S.provisional application Ser. No. 60/337,525, filed Nov. 9, 2001.

FIELD OF THE INVENTION

The present invention is related to the realization of a simplifiedbottom electrode stack for high-density ferroelectric memory cells,where the ferroelectric capacitor is positioned directly on top of thecontact plug.

BACKGROUND OF THE INVENTION

FRAM (Ferroelectric RAM) is random access memory that combines the fastread and write access of dynamic RAM (DRAM)—the most common kind ofpersonal computer memory—with the ability to retain data when power isturned off (as do other non-volatile memory devices such as ROM andflash memory). Because FRAM is not as dense (i.e., cannot store as muchdata in the same space) as DRAM and SRAM, it is not likely to replacethese technologies. However, because it is fast memory with a very lowpower requirement, it is expected to have many applications in smallconsumer devices such as personal digital assistants (PDAs), handheldphones, power meters, and smart card, and in security systems. FRAM isfaster than flash memory. It is also expected to replace EEPROM and SRAMfor some applications and to become a key component in future wirelessproducts.

As depicted in FIG. 2, a ferroelectric memory cell typically comprises aferroelectric capacitor 9 and a selection transistor 2. Theferroelectric capacitor 9 comprises a stack of a conductive bottomelectrode stack 10, a ferroelectric film 11, and a conductive topelectrode 12. The ferroelectric memory cell is programmed by applying anelectrical signal to the conductive top and bottom electrodes across theferroelectric film 11. When an electric field is applied to aferroelectric crystal, the central atom of the ferroelectric compoundmoves in the direction of the field. Internal circuits sense the chargerequired to move the atom. When the electric field is removed from thecrystal, the central atom stays in position, preserving the state of thememory.

The formation of a crystalline ferroelectric film typically requireshigh temperature treatment in oxygen ambient. The film can be preparedby different techniques, such as spin-on, physical vapor deposition(PVD), chemical vapor deposition (CVD), and metal organic chemical vapordeposition (MOCVD). MOCVD may be performed in a two-step process,wherein in a first step the ferroelectric film is deposited at lowertemperature, and afterwards in a second step the ferroelectric film iscrystallized at a higher temperature, e.g. a temperature higher than400° C. in an oxygen ambient. Alternatively, MOCVD may be performed in aone-step process at a higher temperature in oxygen ambient, whereindeposition and crystallization of the ferroelectric film occursimultaneously.

Examples of ferroelectric materials include, but are not limited toSrBi₂Ta₂O₉ (SBT), Pb(Zr,Ti)O₃ (PZT) and (Bi,La)₄Ti₃O₁₂ (BLT). Allferroelectric layers ultimately incorporate oxygen. This oxygen is apart of the so-called “perovskite” crystal structure, which is typicalfor ferroelectric films.

Typically, very complex bottom electrode-barrier structures are used toavoid oxygen diffusion to the plug during the processing of theferroelectric layer, as in a structure identified by layers in the orderPt/IrO₂/Ir(/TiN) wherein TiN is an additional layer which protects thecontact plug from interaction with the electrode stack or improvesadhesion. See, e.g., D. Jung et al., Technical Digest IEDM(International-Electron Devices Meeting), San Francisco, Calif, Dec.10-13, 2000, page 00-801, paper 34.4.1. This TiN layer can be part ofthe contact or can be formed on top of the contact.

The large number of processing steps required to fabricate a complexbottom electrode structure, besides being cost inefficient andenvironmentally unfriendly, imposes stringent requirements on thefabrication of stacked ferroelectric memory cells.

SUMMARY OF THE INVENTION

A method of fabricating ferroelectric memory cells that avoids theoxidation of a metal electrode layer while forming a crystallineferroelectric layer on top of it at elevated temperature in oxygen isdesirable. Also desirable is a method of fabricating a ferroelectricmemory cell wherein a crystalline ferroelectric film is formed on aconductive layer while preserving the conductive properties of thislayer, or a method wherein a crystalline ferroelectric film is formeddirectly on an oxygen diffusion barrier layer. A method of forming aferroelectric capacitor with a simplified bottom electrode stack is alsodesirable.

Accordingly, a method for forming a crystalline ferroelectric layer on ametal electrode in oxygen is provided that avoids the oxidation of theunderlying metal electrode. The method comprises the crystallization ofthe ferroelectric layer in an atmosphere having a reduced oxygen partialpressure. In the method, the total pressure in the process chamber iscontrolled to prevent evaporation of metal or metal oxide compounds fromthe ferroelectric film as it forms. The oxygen partial pressure (pO₂) iskept sufficiently low so as to prevent the oxidation of the metalelectrode, yet sufficiently high so as to prevent the reduction of thechemical elements constituting the ferroelectric film at the processingtemperature. The oxidation of a metal electrode depends not only on theoxygen partial pressure, but also on the processing temperature and onthe reduction potential of the metal electrode. The higher the metalreduction potential is, the higher the minimum temperature at which themetal oxidizes.

The method of preferred embodiments permits the use of a simplifiedbottom electrode barrier structure for stacked ferroelectric memorycells. In a preferred embodiment, the bottom electrode comprises asingle layer, which remains in its metallic form, is conductive, andforms an oxygen diffusion barrier. In a preferred embodiment where Ir isthe bottom electrode and SrBi₂Ta₂O₉ (SBT) is the ferroelectric layer,simple stacks comprising SBT/Ir/contact plug and SBT/Ir/TiN/contact plugmay be formed.

The stacks may be fabricated by forming a bottom electrode layer 10,depositing a ferroelectric layer 11 atop the bottom electrode layer 10,and crystallizing the ferroelectric layer at a temperature (T) in anoxygen ambient, wherein the partial pressure of oxygen in the oxygenambient is controlled at a level sufficient to oxidize the ferroelectriclayer, but not at a level sufficient to oxidize the bottom electrodelayer, wherein the bottom electrode layer is conductive and forms abarrier to oxygen diffusion. The process temperature typically rangesfrom 600° C. to 800° C., and preferably from 650° C. to 750° C. Thepartial oxygen pressure range log(pO₂) is typically from about −3.5 toabout −1.

In a preferred embodiment wherein Ir is the bottom electrode and(Bi,La)₄Ti₃O₁₂ (BLT) is the ferroelectric layer, simple stackscomprising BLT/Ir/contact plug and BLT/Ir/TiN/contact plug may be formedThe process temperature typically ranges from 600° C. to 800° C.,preferably from 650° C. to 750° C. The partial oxygen pressure rangelog(pO₂) is typically from about −3.5 to about −1.

In a preferred embodiment wherein Ru is the bottom electrode andPb(Zr,Ti)O₃ is the ferroelectric layer, simple stacks comprisingPZT/Ru/contact plug and PZT/Ru/TiN/contact plug may be formed. Theprocess temperature typically ranges from 400° C. to 700° C., preferablyfrom 550° C. to 650° C. The partial oxygen pressure range log(pO₂) istypically from about −9 to about −12, at a process temperature rangingfrom 575° C. to 625° C.

In a preferred embodiment, a ferroelectric device is provided, theferroelectric device comprising at least a conductive top electrode 12,a conductive bottom electrode 10, and in between a ferroelectric layer11, the conductive bottom electrode 10 comprises a single substantiallyfree of oxygen layer, being in direct contact with the ferroelectriclayer 11. The single oxygen-free layer is conductive and forms a barrierto oxygen diffusion.

The ferroelectric layer may comprise Pb(Zr,Ti)O₃ (PZT), the bottomelectrode may comprise a single non oxidized layer consisting of Ru andcan further comprise an adhesion layer comprising TiN. The ferroelectriclayer may comprise SrBi₂Ta₂O₉ (SBT), the bottom electrode may comprise asingle nonoxidized layer consisting of Ir and can further comprise anadhesion layer comprising TiN. The ferroelectric layer can comprise(Bi,La)₄Ti₃O₁₂ (BLT), the bottom electrode can comprise a singlenonoxidized layer consisting of Ir and can further comprise an adhesionlayer comprising TiN.

In a first embodiment, a memory cell is provided, the memory cellincluding: a semiconductor chip, the semiconductor chip including acontact; and a capacitor, the capacitor including: a ferroelectric film,a top electrode, and a bottom electrode, wherein the bottom electrodeincludes a single nonoxidized, conductive, oxygen diffusion barrierlayer in contact with the ferroelectric film and the contact.

In an aspect of the first embodiment, the memory cell further includes atransistor, the transistor including a source junction, a drainjunction, a gate, and a channel region

In an aspect of the first embodiment, the single nonoxidized,conductive, oxygen diffusion barrier layer includes ruthenium.

In an aspect of the first embodiment, the single nonoxidized,conductive, oxygen diffusion barrier layer includes iridium.

In an aspect of the first embodiment, the ferroelectric layer includesSrBi₂Ta₂O₉, Pb(Zr,Ti)O₃, and (Bi,La)₄Ti₃O₁₂.

In an aspect of the first embodiment, the contact includes tungsten orpolysilicon.

In an aspect of the first embodiment, the memory cell further includesan adhesion layer in contact with the single nonoxidized, conductive,oxygen diffusion barrier layer and the contact.

In an aspect of the first embodiment, the adhesion layer includes anitride such as Ti nitride, Ta nitride, Al nitride, alloys thereof, ormixtures thereof.

In an aspect of the first embodiment, the contact includes a stack, thestack including titanium nitride on a material such as tungsten oraluminum.

In a second embodiment, a capacitor is provided, the capacitorincluding: a ferroelectric film; a first electrode; and a secondelectrode, the second electrode consisting of a nonoxidized conductive,oxygen diffusion barrier layer.

In an aspect of the second embodiment, the nonoxidized conductive,oxygen diffusion barrier layer includes a single layer, wherein thesingle layer is in contact with the ferroelectric film.

In an aspect of the second embodiment, the memory cell further includesan adhesion layer in contact with the single nonoxidized, conductive,oxygen diffusion barrier layer

In an aspect of the second embodiment, the adhesion layer includes anitride such as Ti nitride, Ta nitride, Al nitride, alloys thereof, ormixtures thereof.

In a third embodiment, a capacitor is provided, the capacitor including:a ferroelectric film; a first electrode; and a second electrode, thesecond electrode including a nonoxidized conductive, oxygen diffusionbarrier layer in contact with the ferroelectric film.

In an aspect of the third embodiment, the memory cell further includesan adhesion layer in contact with the single nonoxidized, conductive,oxygen diffusion barrier layer

In an aspect of the third embodiment, the adhesion layer includes anitride such as Ti nitride, Ta nitride, Al nitride, alloys thereof, ormixtures thereof.

In a fourth embodiment, a method of fabricating a capacitor is provided,the capacitor including a ferroelectric film in contact with aconductive, oxygen diffusion barrier electrode layer, the methodincluding the steps of: forming a conductive, oxygen diffusion barrierelectrode layer; depositing a ferroelectric layer atop the conductive,oxygen diffusion barrier electrode layer; and annealing theferroelectric layer in an oxygen ambient, wherein a partial pressure ofoxygen in the oxygen ambient pO₂ is controlled at a level sufficient tooxidize the ferroelectric layer but not at a level sufficient to oxidizethe conductive, oxygen diffusion barrier electrode layer.

In an aspect of the fourth embodiment, the conductive, oxygen diffusionbarrier electrode layer includes ruthenium.

In an aspect of the fourth embodiment, the conductive, oxygen diffusionbarrier electrode layer includes iridium.

In an aspect of the fourth embodiment, a log(pO₂) is greater than alog(pO₂-Bulk), wherein pO₂-Bulk is a partial pressure of oxygen in abulk N₂ gas containing approximately 0.07 ppm O₂.

In an aspect of the fourth embodiment, a log(pO₂) is from about −3.5 toabout −1.

In an aspect of the fourth embodiment, the annealing is conducted at atemperature of from about 600° C. to about 800° C.

In an aspect of the fourth embodiment, the annealing is conducted at atemperature of from about 650° C. to about 750° C.

In an aspect of the fourth embodiment, the annealing is conducted at atemperature of from about 667° C. to about 717° C.

In an aspect of the fourth embodiment, the conductive, oxygen diffusionbarrier electrode layer is iridium, and the annealing is conducted at atemperature of from about 667° C. to about 717° C. and a pO₂ of fromabout 0.0532 mtorr to about 2.81 torr.

In an aspect of the fourth embodiment, the conductive, oxygen diffusionbarrier electrode layer is ruthenium, and the annealing is conducted ata temperature of from about 400° C. to about 600° C. and a log(pO₂)(atm) of from about −12.5 to about −9.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides a schematic graph of Log(pO₂)-T (i.e., logarithmic ofthe partial oxygen pressure as function of the absolute temperature) fora ferroelectric layer, an Ir bottom electrode layer, and a Pt bottomelectrode layer.

FIG. 2 provides a cross-sectional view of a memory cell showing asemiconductor substrate 1, an insulating layer 7, a contact 8, and aferroelectric memory cell (or capacitor) 9 including a bottom electrode10, a ferroelectric layer 11, and a top electrode 12. The memory cell 9is stacked on a selection transistor 2, including a source junction 3, adrain junction 4, a gate 5, and a channel region 6.

FIG. 3 provides a Log(pO₂)-T diagram for the different metal/metal in anSBT/Ir system.

FIG. 4 a provides stability curves for Ir and Bi, with experimental datapoints for Ir, IrO₂, and SBT stability.

FIG. 4 b provides stability curves for Ir, Bi, La, with experimentaldata points for Ir, IrO₂, and LBT stability.

FIG. 5 provides a Log(pO₂)-T diagram for the different metal/metal oxideelements in a PZT/Ru system.

FIG. 6 provides a cross-sectional SEM image of an Ir/TiN sample annealedat 700° C., 30 min in 100% O₂. Lines delineating the different layershave been added to the micrograph.

FIG. 7 provides a cross-sectional SEM image of an Ir/TiN sample annealedat 700° C., 30 min in 10 ppm O₂ in N₂. Lines delineating the differentlayers have been added to the micrograph.

FIG. 8 provides a cross-sectional SEM image of two SBT/Ir/TiO₂ samplesannealed at 700° C., 30 min in 100 ppm O₂ in N₂. Lines delineating thedifferent layers have been added to the micrograph.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The following description and examples illustrate a preferred embodimentof the present invention in detail. Those of skill in the art willrecognize that there are numerous variations and modifications of thisinvention that are encompassed by its scope. Accordingly, thedescription of a preferred embodiment should not be deemed to limit thescope of the present invention.

In conventional stacked ferroelectric memory cells, the use of anelectrode layer comprising solely, for example, Ir or Ru is typicallynot feasible, even though these metals are expected to have good oxygenbarrier properties. In fact, these metals oxidize in an uncontrolled wayduring processing of the ferroelectric layer deposited on top of themwhen conventional conditions of elevated temperature and an oxygenambient are employed.

The stability of Ir and Ru under oxidative conditions compared to thestability of Pt is illustrated by the schematic graph of FIG. 1. They-axis corresponds to log (pO₂) (atm) and the x-axis corresponds totemperature (T, in Kelvin). FIG. 1 provides a schematic graph showingthe logarithmic of the partial oxygen pressure as function of theabsolute temperature for a ferroelectric layer (the curve labeled “Bi”),an Ir bottom electrode layer (the curve labeled “Ir”), and a Pt bottomelectrode layer (the curve labeled “Pt”). As is depicted in the graph,Ir is more sensitive to oxidation than Pt, as illustrated by the factthat the stability curve of Ir corresponds to a much lower oxygenpartial pressure than the stability curve of Pt. A smaller processwindow of acceptable partial oxygen pressure conditions (“Δp” in FIG. 1)at a given temperature (“T₆₆ p” in FIG. 1) is therefore available for Ircompared to Pt.

The formation of the ferroelectric film and/or the crystallization ofthe ferroelectric film take place in a controlled environment, i.e., aprocess chamber. A method for forming a crystalline ferroelectric layeris provided that avoids the oxidation of the underlying electrode, suchthat a simplified electrode/barrier structure can be employed. Themethod involves the crystallization of the ferroelectric layer in areduced oxygen partial pressure. An advantage of this method is that abottom electrode 10 (as depicted in FIG. 2) can be formed comprising anon-oxidized layer in contact with the ferroelectric layer 11, whilethis nonoxidized or metallic layer is conductive and forms an oxygendiffusion barrier.

In the fabrication of a ferroelectric capacitor, the ferroelectric film11 is sandwiched between a top electrode 12 and a bottom electrode stack10. In general the bottom electrode stack fulfills several requirements.The parts of the bottom electrode exposed to the oxygen ambient arepreferably stable in oxygen at high temperature or form a conductiveoxide after exposure of the electrode material to an oxygen ambient.Suitable materials include noble metals such as platinum and conductiveelectrode materials such as IrO₂ and RuO₂. In this manner, the bottomand top electrode layers remain conductive and an electrical signal canbe conveyed to the ferroelectric film in order to program the memorycell.

In stacked ferroelectric memory cells, the ferroelectric capacitor 9 ispreferably placed on top of a contact 8 in order to conserve area. Thecontact can be formed from a stack of layers. These layers, however, arenot considered part of the bottom electrode stack 10 of the capacitorstructure itself, as these contact layers 8 are used for contacts on thechip and to contact the bottom electrode stack 10. The contact 8connects the memory capacitor 9 with the selection transistor 2. Thecontact 8 comprises, for example, a plug fill material 81 such astungsten or polysilicon, and can further comprise an adhesion layer 82on top of the plug fill material 81. This adhesion layer 82 canfurthermore prevent interaction of material of the bottom electrodestack 10 with contact material 81, e.g., prevent the formation of asilicide due to the interaction of Ir from the bottom electrode with Siof the plug fill material 81. This adhesion layer consists of, e.g.,nitrides of Ti, Ta, Al, or alloys thereof. The transistor 2 and thecontact 8 are commercially available on chips. It is desirable that thecharacteristics of these elements as used in the “digital” or “logic”circuitry on a chip are not influenced by the formation of the memorycells in subsequent processing.

The bottom electrode stack 10 is conductive when exposed to a hightemperature oxygen containing ambient. The bottom electrode stack 10forms a barrier to diffusion of oxygen from the oxygen containingambient towards the underlying layers, such as the contact 8, in orderto avoid oxidation of the materials used to form the contact as anon-conductive layer would then be formed. The bottom electrode stack 10does not react, e.g. oxidize, the underlying layers, such as the layersof the contact 8.

Because of these additional requirements, multiple layers, each from adifferent material, are used to constitute the bottom electrode inconventional methods. Namely, the well-studied Pt electrode (which doesnot oxidize under typical process conditions) cannot be used alonebecause of its insufficient oxygen barrier properties. In contrast, itis known that IrO₂ on top of TiN results in the formation of a TiO₂interfacial layer. On the other hand, metal oxide films such as IrO2 andRuO₂, which have good oxygen diffusion barrier properties, act aspowerful oxidizers when in contact with a plug or with an adhesionmaterial like TiN. Hence, the addition of a metallic Ir or Ru layerunderneath the IrO₂ or RuO₂ layer to separate the IrO₂ or RuO₂ layerfrom the contact material 8 is required in conventional methods.Moreover, the use of layers comprising solely Ir or Ru, which arebelieved to be good oxygen barrier layers, is not attempted inconventional methods because such layers oxidize in an uncontrolled wayduring the processing in oxygen of the ferroelectric layer deposited ontop of them.

In the method of preferred embodiments, the total pressure in theprocess chamber is set so as to prevent evaporation of metal or metaloxide compounds from the ferroelectric film. For example, in a two-stepprocess, a fixed total pressure of about 1 atm can be employed. Duringthe second step of the two-step process, i.e., the annealing step, thepartial oxygen pressure (pO2) range is selected as a function of theannealing temperature such that the electrode metal (e.g., Ir or Ru) isnot oxidized, thus defining the upper bound of the selected range, andsuch that the metal compounds constituting the ferroelectric layer donot undergo any chemical reduction, thus defining the lower bound of theselected range of the oxygen partial pressure. The method is alsoapplicable for a one-step process wherein deposition of theferroelectric film and crystallization of this film occurssimultaneously because of the higher deposition temperature and thepresence of the oxygen in the process chamber. FIG. 3 provides an oxidestability curve (or metal oxide decomposition curve) as a function ofoxygen partial pressure and temperature for a representative systemwherein the bottom electrode is Ir and the ferroelectric material isSrBi₂Ta₂O₉ (SBT). The y-axis corresponds to log (pO₂) (atm) and thex-axis corresponds to temperature (T, in Kelvin). The upper and lowerbounds of the oxygen partial pressure range for a given temperaturerange are identified in FIG. 3 by the process window.

The oxidation of a metal electrode is a function not only of the oxygenpartial pressure, but also of the processing temperature and thereduction potential of the metal electrode. The higher the metalreduction potential is, the higher the minimum temperature at which themetal electrode oxidizes. Generally, annealing temperatures from about400° C. or lower to about 800° C. or higher are preferred.

For a given metal; it is possible to determine from thermodynamiccalculations a good estimate of the oxygen partial pressures as afunction of temperature above which a metal oxide is stable and belowwhich the reduced metal form is stable. These calculations can be basedon Richardson-Ellingham diagrams that show the relative stability versustemperature for different metal oxides at 1 atm total pressure. See,e.g., “Thermodynamics in Material Science,” R. T. De Hoff, McGraw Hill,Inc, 1993. These data can be recalculated into an oxide stability curve(or metal oxide decomposition curve) in an oxygen partial pressureversus temperature diagram. FIG. 3 provides such a diagram for thespecific case wherein the bottom electrode is Ir and the ferroelectricmaterial is SrBi₂Ta₂O₉ (SBT). FIG. 3 provides stability curves as afunction of log (pO₂) (atm) over a temperature range of from 300 to 1200K. The stability curve for IrO₂ (represented by the line with diamonds)begins at about 4.times.E.sup.−33 at 300K and increases to about1.4.times.E.sup.−1 at 1200K. The stability curve for 2/3(Bi₂O₃)(represented by the line with squares) begins at about 2.times.E.sup.−58 at 300K and increases to about 1.times.E.sup.−8 at 1200K.The stability curve for 2(SrO) (represented by the line with triangles)begins at about 1.4.times.E.sup.−196 at 300K and increases to about7.times.E.sup.−42 at 1200K. The stability curve for 2/5(Ta₂O₅)(represented by the line with x's) begins at about 8.times.E.sup.−134 at300K and increases to about 7.times.E.sup.−27 at 1200K. Although someapproximations are made in the calculation of such curves, it isobserved that IrO₂ is less stable than any of the metal oxides formingthe complex SBT oxide, as shown by the stability curve of Ir/IrO₂ inFIG. 3, which lies above all the others. Bi₂O₃ is the least stable oxideconstituting the complex SBT oxide, as shown by the stability curve ofBi/Bi₂O₃ in FIG. 3 which lies above the ones for Ta/Ta₂O₅ and Sr/SrO.The data displayed in FIG. 3 is provided in tabular form in thefollowing Table 1.

1TABLE 1 pO2=(EXP(1n(pO2))T(K)IrO₂ 2/3(Bi₂O₃) 2(SrO) 2/5(Ta₂O₅) 300.004.12E-33 2.28E-58 1.42E-196 8.38E-134 400.00 1.34E-22 8.29E-42 5.20E-1453.71E-98 500.00 2.72E-16 7.16E-32 4.50E-114 9.05E-77 600.00 4.36E-123.02E-25 1.90E-93 1.64E-62 700.00 4.39E-09 1.63E-20 1.03E-78 2.51E-52800.00 7.86E-07 5.75E-17 1.15E-67 1.09E-44 900.00 4.44E-05 3.31E-144.51E-59 9.52E-39 1000.00 1.12E-03 5.35E-12 3.38E-52 5.39E-34 1100.001.57E-02 3.43E-10 1.43E-46 4.17E-30 1200.00 1.42E-01 1.10E-08 6.95E-427.26E-27

Focusing only on the curves for Ir/IrO₂ and Bi/Bi₂O₃ in FIG. 3, andbased on improved approximations (see, e.g., CRC, 77.sup.th edition, CRCpress; S. Yong Cha and H. Chul Lee, Jpn. J. Appl. Phys, Vol. 38 (1999),page 1128), the relevant curves are recalculated in FIG. 4 a. The y-axiscorresponds to log (pO₂) (torr) and the x-axis corresponds totemperature (T, in degrees Celsius). The stability curve for Ir(represented by the line with small diamonds) is a straight linebeginning at about −1.5 at 600° C. and increasing to about 1 at 800° C.The stability curve for 2/3(BiO₂) (represented by the line with smalltriangles) is a straight line beginning at about −7 at 600° C. andincreasing to about −3.5 at 800° C. The partial pressure of O₂ in bulkN₂ (represented by the heavy solid line) is a level straight line atabout −4.5 from 600° C. and to 800° C. Specific temperatures and partialpressures of O₂ at which IrO₂ is stable include a pO₂ of −2 at 650° C.,700° C., and 750° C. pO₂ of −1.5 at 700° C. (all identified by largediamonds). Specific temperatures and partial pressures of O₂ at which Iris stable include a pO₂ of 3 at 650° C., 700° C., and 750° C., and a pO₂of 2 at 700° C. (all identified by large circles). A specifictemperature and partial pressure of O₂ at which SBT is reduced is −4.5at 750° C. (identified by a large gray triangle). The Ir stability curveas calculated by Yong & Chul is depicted as the straight dotted linebeginning at about −1.75 at 600°° C. and ending at about 0.5 at 800° C.See S. Yong Cha, H. Chul Lee, Japanese Journal of Applied Physics, vol.38, 1999, p1128. In FIG. 4 a at 700° C. which is the temperatureconventionally used for the crystallization of SBT films, there is aworking window for pO₂ wherein both Ir and Bi₂O₃ are stable. This windowconsists of the partial pressures of oxygen below the Ir/IrO₂ (curve)and above the Bi/Bi₂O₃ curve.

FIG. 4 b provides stability curves for Ir, Bi, and La, with experimentaldata points for Ir, IrO₂, and LBT stability. The y-axis corresponds tolog (pO₂) (torr) and the x-axis corresponds to temperature (T, indegrees Celsius). The stability curve for Ir (represented by the heavyblack curve) begins at about −5 at 375° C. and increases to about 0 at800° C. The stability curve for Bi (represented by the heavy gray curve)La (represented by the heavy light gray curve) begins at about −57 at600° C. and increases to about −43 at 800° C. The partial pressure of O₂in bulk N₂ (represented by the level heavy solid line) is a levelstraight line at about −4.5 from above 600° C. to below 800° C. Specifictemperatures and partial pressures of O₂ at which IrO₂ is stable includea pO₂ of −2 at 650° C., 700 C., and 750° C., and a pO₂ of −1.5 at 700°C. (all identified by light gray circles). Specific temperatures andpartial pressures of O₂ at which Ir is stable include a pO₂ of 3 at 650°C., 700° C., and 750° C., and a pO₂ of 2 at 700° C. (all identified bygray circles). The Ir stability curve as calculated by Yong & Chul isdepicted as a dotted line overlapping the Ir curve depicted by the heavygray curve. As the stability curve for La is below the stability curveof Bi, Bi is more likely then La to form a metal oxide. For the LBT/Irsystem, the process window is determined by stability curves of Ir andBi.

FIG. 5 provides such a diagram for the specific case wherein the bottomelectrode is Ru and the ferroelectric material is Pb(Zr,Ti)O₃ (PZT). They-axis corresponds to log (pO₂) (atm) and the x-axis corresponds totemperature (T, in degrees Celsius). FIG. 5 provides stability curves asa function of log (pO₂) (atm) over a temperature range of from 400° C.to 850° C. The stability curve for RuO₂ (represented by the line withdiamonds) begins at about −15 at 400° C. and increases to about −5 at850° C. The stability curve for 2/3(Bi₂O₃) (represented by the line withsquares) begins at about −17 at 400° C. and increases to about −6 at850° C. The stability curve for 2(PbO) (represented by the line withcircles) begins at about −20 at 400° C. and increases to about −7 at850° C. For comparison purposes, the stability curve for IrO₂(represented by the line with hollow rectangles) begins at about −8 at400° C. and increases to about −2 at 850° C. As illustrated in FIG. 5,the preferred process window is one wherein partial pressure of oxygenis preferably from about −12.5 to about −9 ΔpO₂, more preferably fromabout −12 or −11 to about −10 or −9. Temperature is preferably fromabout 400° C. to about 600° C., more preferably from about 425, 450,475, or 500° C. to about 525, 550, or 575° C. However, in certainembodiments, partial pressures of oxygen below −12.5 or above −9, and/ortemperatures below about 400° C. or above about 600° C. may bepreferred. Most preferred is the process window wherein partial pressureof oxygen is from about −12.5 to about −9 and the temperature is fromabout 400° C. to about 600° C.

It is generally preferred that the pO₂ pressure range is above the pO²present in bulk (i.e., so-called pure) N₂, as used in a typicalsemiconductor fabrication. If the process window requires a partialoxygen pressure below this practical limit, then industrial processingmight be difficult. FIG. 5 provides stability curves for Ru, Pb as usedin a PZT/Ru system. In this case, the process window is below the pO₂present in bulk (i.e., so-called pure) N₂. FIG. 5 provides such adiagram for the specific case wherein the bottom electrode is Ru and theferroelectric material is Pb(Zr,Ti)O₃ (PZT). In this case, other gasesare used or added to obtain such a low partial pressure of oxygen.Examples of such gasses include reducing gasses, such as CO, as will beappreciated by a person skilled in the art. A process window for aPZT/Ru system typically includes an oxygen partial pressure range of log(pO₂) from about −1.5 to about −3. A process window for an SBT/Ir systemtypically includes a temperature range from about 650° C. to about 750°C. At a process temperature T_(Δ)pO₂, the oxygen partial pressure rangeΔpO₂ of log(pO₂) is typically from about −9 to about −12.

Several experiments were performed to define such a working oxygenpartial pressure window. The following conditions and materials wereemployed in the experiments: an SBT ferroelectric layer, an Ir bottomelectrode layer, an Ir/TiO₂ or Ir/TiN bottom electrode-adhesionstructure, a crystallization temperature of 650° and 750° C., and anoxygen partial pressure during crystallization of 100% O₂, 10% O₂ in N₂(prepared by mixing O₂ and N₂ gas), 100 ppm O₂ in N₂ (using a premixedgas mixture), 10 ppm O₂ in N₂ using 10% of a 100 ppmO₂ in N₂ gas mixturein 90% N₂, and 100% N₂ (flow limited by the oxygen content of bulk N₂ toapproximately 0.07 ppm O₂).

Simple Ir/TiO₂ or Ir/TiN stacks were annealed at a temperature between650 and 750° C. with different oxygen partial pressures. Note that theIr/TiO₂ stack is the product of the formation of an Ir layer on analready formed TiO₂ bottom electrode layer. This TiO₂ bottom electrodelayer is not the product of the oxidation of a Ti layer during theformation of a PZT layer.

Experimental data points indicating the presence of Ir or IrO₂ arereported in FIG. 4 a. The stability of IrO₂ at 10% O₂ is confirmed atall of the reported temperatures by Auger spectroscopy. Scanningelectron microscopy (SEM) investigations on Ir(10)/TiN(81)/bulk(1)stacks annealed in 100% O₂ at 700° C. revealed large non-uniformIrO₂(13) grains, as depicted in FIG. 6. FIG. 7 shows that formation ofIrO₂ grains is not observed when Ir(10)/TiN(81)/bulk(1) is annealed atthe same temperature in 10 ppm O₂. Similar results are observed forannealing at 100 ppm O₂. An optimal window of temperature and partialpressure of O₂ (pO₂) for Ir may be determined from the data. Assuming anambient pressure of 760 torr (1 atm), a pO₂ exceeding that of a bulk N₂gas (containing approximately 0.07 ppm O₂) is preferred. However, incertain embodiments, a nonzero partial pressure of oxygen less than thatobserved in a bulk N₂ gas as described above may be acceptable. The pO₂is preferably within the range at which Ir is stable, but less than thepO₂ at which IrO₂ is stable, at the specified temperature. A processwindow for an SBT/Ir system preferably includes a temperature of fromabout 600° C. to about 800° C., more preferably from about 625° C. or650° C. to about 750° C. or 775° C., and most preferably from about 650°C. or 675° C. to about 700° C., 725° C., or 750° C. However, in certainembodiments a process window including a temperature of below about 600°C. or above about 800° C. may be preferred. A process window for anSBT/Ir system preferably includes an oxygen partial pressure, in unitsof log (pO₂), of from about −1.5 to about −3, more preferably from about−1.75 or −2 to about −2.75 or −3, and most preferably about −2.5.However, in certain embodiments a log(pO₂) of greater than about −1.5 orless than about −3 may be preferred. A particularly preferred processwindow for an SBT/Ir system includes a temperature from about 650° C. toabout 750° C., and an oxygen partial pressure, in log (pO₂) of fromabout −1 and about −3.5. Even more particularly preferred processconditions for Ir include a temperature of between about 667° C. and717° C., a total pressure of about 760 torr, and a partial pressure ofoxygen of from about 0.0532 mtorr to about 2.81 torr. If it is preferredto conduct the process at a temperature of less than about 667° C. or attemperature greater than about 717° C., the partial pressure of oxygenmay be adjusted up or down accordingly so as to remain at a nonzerovalue below the Ir stability limit (depicted by the smaller diamonds inFIG. 4 a) at that temperature. Preferred combinations of partialpressure of oxygen and temperature may also be determined for Ru usingthe same methodology once the stability limit is determined. Thepreferred combination will depend upon the particular metal selected.

An SBT layer 11 was deposited on top of the bottom electrode stack. AnSBT(11)/Ir(10)/TiO₂(82)/bulk(1) sample annealed in 1 atm O₂ shows asubstantial amount of interfacial IrO₂ (13) to be formed. In contrast,for anneals at 10-100 ppm O₂, a substantially oxide free SBT(11)/Ir(10)interface is maintained, as shown in FIG. 8. From these results, it canbe concluded that the presence of an SBT layer does not prohibit theoxidation of the Ir electrode and that the low pO₂ is responsible forthe formation of a controlled SBT/Ir structure. For the SBT/Ir samples,no optically visible change in the SBT material occurred for anneals inthe different pO₂ ambients, except for the anneal at 750° C. in a 100%N₂ flow (the triangle in FIG. 4 a). At the latter condition, a graymetallic shine appeared at the surface of the structure. As expectedfrom thermodynamical calculations, the Bi₂O₃ layer is expected to bereduced, and, because of the high temperature, the Bi is expected tomelt. From merely theoretical calculations, it is impossible todetermine the exact position of the metal oxide stability curves shownin FIGS. 4 a and 4 b. A partially reduced form of Bi₂O₃, e.g. BiO, mayform at oxygen partial pressures at which Bi₂O₃ is predicted bytheoretical calculations to be stable. The existence of intermediatephases, such as fluorite or pyrochlore, is well known and they mayinfluence the nucleation mechanism. In view of these considerations, thelower bound for pO₂ is preferably controlled in such a way as to preventreduction of the metal constituting the ferroelectric material.

The above-described method may be employed to fabricate ferroelectriccapacitors of the type Ir/SBT/Ir with hysteresis characteristicscomparable to those of a Pt/SBT/Pt ferroelectric capacitor. Annealingferroelectric films has been proposed to improve the film morphology,structure, or texture. See Fujimori et al., “Low-temperaturecrystallization of sol-gel-derived Pb(Zr,Ti)O₃ thin films” in theJapanese Journal of Applied Physics Vol 38, 1999, p5346-5349, where areduced partial oxygen pressure is used to promote outgassing of thesolvents used during spin coating of the sol-gel. In other publicationswherein it has been reported to anneal SBT films at reduced oxygen totalpressure with the goal of improving film quality and loweringcrystallization temperature, a large loss of Bi element was reported.See Ito, Jpn. J. Appl. Phys., Vol. 35 (1996), pp. 4925-4929; and Ogataet al., Extended Abstracts of the 1997 International Conference on SolidState Devices and Materials, Hamamatsu, 1997, page 40-41.

In contrast, a reduced partial pressure of oxygen and a fixed totalpressure in the methods of preferred embodiments is employed to avoidthe oxidation of the underlying electrode metal, and ultimately topermit the use of a simplified bottom electrode barrier structure forstacked ferroelectric memory cells.

In a preferred embodiment as described in detail above, wherein Ir isthe bottom electrode and SBT is the ferroelectric layer, the followingsimple stacks can be prepared: SBT/Ir/contact plug; andSBT/Ir/TiN/contact plug. As depicted in the cross-sectional SEMs frominvestigations of oxygen-annealed Ir/TiN samples formed on a siliconwafer 1 as shown in FIG. 6 and FIG. 7, no oxidation of the TiN(81) layeris observed for anneals at 1 atm total pressure and 100% O₂ and 10 ppmO₂. These results demonstrate that Ir exhibits good oxygen barrierproperties. Because of the low oxygen partial pressure employed in thecrystallization process described above, layers comprising Ir alone canserve as electrode materials and barrier layers. Often in the formationof ferroelectric stacked memory cells, one or more additional layers,e.g. TiN, are placed between the electrode material and the contact plugin order to improve adhesion or to avoid material interaction.

The above description discloses several methods and materials of thepresent invention. This invention is susceptible to modifications in themethods and materials, as well as alterations in the fabrication methodsand equipment. Such modifications will become apparent to those skilledin the art from a consideration of this disclosure or practice of theinvention disclosed herein. Consequently, it is not intended that thisinvention be limited to the specific embodiments disclosed herein, butthat it cover all modifications and alternatives coming within the truescope and spirit of the invention as embodied in the attached claims.All patents, applications, and other references cited herein are herebyincorporated by reference in their entirety.

1. A method of fabricating a capacitor, the capacitor comprising aferroelectric film in contact with a conductive, oxygen diffusionbarrier electrode layer, the method comprising the steps of: forming aconductive, oxygen diffusion barrier electrode layer; depositing aferroelectric layer atop the conductive, oxygen diffusion barrierelectrode layer; and annealing the ferroelectric layer in an oxygenambient, wherein a partial pressure of oxygen in the oxygen ambient pO₂is controlled at a level sufficient to oxidize the ferroelectric layerbut not at a level sufficient to oxidize the conductive, oxygendiffusion barrier electrode layer.
 2. The method of claim 1, wherein theconductive, oxygen diffusion barrier electrode layer comprisesruthenium.
 3. The method of claim 1, wherein the conductive, oxygendiffusion barrier electrode layer comprises iridium.
 4. The method ofclaim 1, wherein a log(pO₂) is greater than a log(pO₂-Bulk), whereinpO₂-Bulk is a partial pressure of oxygen in a bulk N₂ gas containingapproximately 0.07 ppm O₂.
 5. The method of claim 1, wherein a log(pO₂)is from about −3.5 to about −1.
 6. The method of claim 1, wherein theannealing is conducted at a temperature of from about 600° C. to about800° C.
 7. The method of claim 1, wherein the annealing is conducted ata temperature of from about 650° C. to about 750° C.
 8. The method ofclaim 1, wherein the annealing is conducted at a temperature of fromabout 667° C. to about 717° C.
 9. The method of claim 1, wherein theannealing is conducted at a temperature of from about 667° C. to about717° C. and a pO₂ of from about 0.0532 mtorr to about 2.81 torr.
 10. Themethod of claim 3, wherein the annealing is conducted at a temperatureof from about 667° C. to about 717° C. and a pO₂ of from about 0.0532mtorr to about 2.81 torr.
 11. The method of claim 2, wherein theannealing is conducted at a temperature of from about 400° C. to about600° C. and a log(pO₂) (atm) of from about −12.5 to about −9.